Add a fallback test workflow. (#2971)

To make this cleaner, this switches the main `test` workflow to use
exclusion path patterns as that was the clearly documented intent
anyways.

This works around a problem with required GitHub status checks:
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore

Without something like this, we won't be able to use the merge queue (or
require passing tests).

.github/workflows/tests-fallback.yaml

@@ -0,0 +1,46 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# Note that this name must exactly match the name in `tests.yaml`.
+#
+# This is not a real "test" workflow, but a fallback used to work around a
+# problem where ignored checks cannot be required:
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore
+name: test
+
+# This mus exactly match the `on` clause in `tests.yaml` except for inverting
+# `paths-ignore` to `paths`. The purpose is that exactly one of this workflow or
+# the one in `tests.yaml` fires for a given case.
+on:
+  push:
+    branches: [trunk]
+    paths:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
+  pull_request_target:
+    paths:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
+  merge_group:
+    paths:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
+
+# Cancel previous workflows on the PR when there are multiple fast commits.
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - run: 'echo "No tests to run."'

.github/workflows/tests.yaml

@@ -7,32 +7,23 @@ name: test
 on:
   push:
     branches: [trunk]
-    paths:
-      # Conservatively run the tests. However, skip them if the only paths in
-      # the pull request match files that we know don't impact the build.
-      - '**'
-      - '!**/*.md'
-      - '!LICENSE'
-      - '!CODEOWNERS'
-      - '!.git*'
+    paths-ignore:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
   pull_request_target:
-    paths:
-      # Conservatively run the tests. However, skip them if the only paths in
-      # the pull request match files that we know don't impact the build.
-      - '**'
-      - '!**/*.md'
-      - '!LICENSE'
-      - '!CODEOWNERS'
-      - '!.git*'
+    paths-ignore:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
   merge_group:
-    paths:
-      # Conservatively run the tests. However, skip them if the only paths in
-      # the pull request match files that we know don't impact the build.
-      - '**'
-      - '!**/*.md'
-      - '!LICENSE'
-      - '!CODEOWNERS'
-      - '!.git*'
+    paths-ignore:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
 
 # Cancel previous workflows on the PR when there are multiple fast commits.
 # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency